There is a constant demand for improved, lightweight high temperature materials for use in gas turbine engines typically used in aircraft. Much effort has been directed to superalloys based on iron, nickel and cobalt. However, another area having a great amount of potential is that of intermetallic compounds.
Intermetallic compounds, frequently referred to simply as intermetallics, are compounds of metals having particular crystal structures which are different from those of the component metals. Intermetallics have ordered atom distribution. Although the bonding of intermetallics is still predominantly metallic bonding, making them less brittle than ceramics, they still tend to be brittle at ambient temperature. These ordered structures exist over specific composition ranges and exhibit high melting points while having the potential for good strength, despite having low ductilities or fracture toughnesses at ambient temperature. Typical intermetallics include TiAl, Ti.sub.3 Al, Ni.sub.3 Al and NiAl.
The NiAl system is of particular interest. It is particularly attractive for use as a turbine airfoil. These airfoils typically are made from nickel base superalloys. However, NiAl intermetallics offer reduced density, up to 33% lower, and higher thermal conductivity, up to 300%, as compared to nickel base superalloys. However, the low ductility of NiAl intermetallics, less than 1% between room temperature and about 600.degree. F., has impeded the implementation of NiAl intermetallics as a viable substitute for nickel base alloys.
Although many investigations have been directed to improvements and refinements in Ni.sub.3 Al, investigations into improvements in NiAl have been somewhat limited. For example, Liu et al., in U.S. Pat. Nos. 4,612,165 and 4,731,221, have investigated ductility improvements in Ni.sub.3 Al having less than 24.5% by weight of aluminum by additions of effective amounts of boron plus additions of 0.35 to 1.5% of hafnium, zirconium, iron and combinations thereof. Similarly, Huang et al., in U.S. Pat. No. 4,478,791, explored improvements in the ductility of Ni.sub.3 Al intermetallics by additions of small amounts of boron.
The NiAl intermetallic system has also been studied. Most work has been directed to improving ambient temperature ductility of NiAl. Law et al, in U.S. Pat. No. 4,961,905 have investigated improvements in the ductility and toughness of the intermetallic at low temperatures by adding at least 10 at. % cobalt in order to cause the formation of the Ll.sub.0 martensitic phase. Rudy and Sauthoff, in their paper, "Creep Behaviour of the Ordered Intermetallic (Fe,Ni)Al Phase", Mat. Res. Soc. Symp. Proc., Vol. 39 (1985), discuss creep behavior of NiAl intermetallics containing at least 10 at. % iron, and conclude that the creep resistance of these brittle alloys is at a maximum at 10 at. % iron.
Law and Blackburn have studied the effects of gallium additions in poycrystalline NiAl. In their Final Air Force Report AFWAL-TR-87-4102 (December 1987) entitled "Rapidly Solidified Lightweight Durable Disk Material", gallium contents as low as 0.5% were added to beta NiAl, with no ductility improvements being observed in polycrystalline NiAl.
Barrett et al., U.S. Pat. No. 4,610,736, added small amounts of zirconium, 0.05% to 0.25% by weight, to NiAl to improve the cyclic oxidation resistance of NiAl as a coating. Grala et al. report in "Investigations of NiAl and Ni.sub.3 Al", Mechanical Properties of Intermetallic Compounds, (1960) that additions of 0.5% by weight molybdenum resulted in a heavy grain boundary precipitate, but lowered the brittle-ductile transition temperature of NiAl to room temperature, thereby improving ductility to about 1.9%.
It would be desirable if intermetallic compounds could be alloyed in a manner so as to improve the room temperature ductility of NiAl intermetallics, while maintaining the ordered atomic structure of the intermetallic, which contributes to such desirable characteristics as high temperature tensile strength, high melting point and excellent thermal conductivity.